1. Field of the Invention
This invention generally relates to an alkaline nickel oxide-zinc cell and more particularly, to a primary alkaline nickel oxide-zinc cell containing a mixture of nickel oxide and manganese dioxide as the positive electrode material, wherein, preferably, an electroconductive sheet containing many apertures such as a wire netting, tightly covers the surface of the positive electrode. The alkaline cells with this positive electrode configuration are characterized by improved stability during the storage or shelf life of the cells and an improved capacity even under heavy current discharge loads.
2. Description of the Prior Art
It is well known that nickel oxide-zinc cells have high terminal voltage, flat voltage-time discharge characteristics and a high available capacity even at low temperature. However, these cells have the disadvantages of having an insufficient capacity under heavy discharge current loads, particularly, under intermittent heavy current loads. In addition, the capacity of the cells substantially reduces during storage of the cells.
In the battery art, several methods have been used in attempts to overcome these disadvantages. For example, the negative electrode has been improved by dispersing finely divided zinc particles or fibered zinc particles in a gel matrix which is prepared by mixing an alkaline electrolyte with a gel forming material and a metallic oxide stable under alkaline conditions. With this procedure, substantial improvement in the capacity of the negative electrodes has been achieved even under the influence of heavy current loads, particularly at temperatures as low as -10.degree.C.
Other attempts to improve the cells have been concerned with improvements in the separator whose function is to prevent a short circuit between the negative and positive electrodes. These improvements had been achieved by the use of Cellophane sheets, porous plastic sheets or sheets of nonwoven plastic cloth durable to caustic alkali.
Still other attempts to improve the cells have been concerned with improvements in the positive electrode characteristics which have been achieved by impregnating an active nickel oxide into a porous nickel matrix which is made by sintering finely divided metallic nickel. The method is effective in increasing the capacity of the cell available under heavy current loads. However, the capacity of the cells containing this type of positive electrode during storage is seriously reduced because of the large self discharge of the cells. This disadvantage is too serious a handicap to use cells containing sintered nickel electrodes practically as primary cells.
In order to overcome the above mentioned disadvantage of the positive electrode, an alternative method of construction of the positive electrode has been provided, wherein pulverized nickel oxide is mixed with an electroconductive material such as graphite and a binder such as Portland cement or thermoplastics durable to oxidation in alkaline media. This method effectively prevents to some extent, the reduction of the capacity of the cell during storage. However, this advantage is achieved at the expense of reduced cell capacity available under heavy current loads, since the positive electrode material expands locally during heavy current discharge which results in poor conductivity due to loosening of the electrode material. An attempt has been made to improve the cycle life of secondary nickel-zinc cells by wrapping the positive electrode material with metallic wire netting. This method is effective in preventing loosening and collapsing of the formed electrode material. However, this method is ineffective in improving the capacity of primary cells under heavy current loads.
Still another method of improving the positive nickel electrode of a secondary cell has been tried, wherein the electrode is composed of a composition of nickel oxide and cobalt oxide. This method, however, is also ineffective in improving the capacity of primary cells when discharged under heavy current loads. On the other hand, when the positive electrode is composed of an active mixture of materials comprisng 100 parts of manganese dioxide and less than 100 parts of nickel oxide, the cycle life of the cells improves significantly compared to cells wherein only manganese dioxide is used as the positive electrode material. This method, however, is effective for use only in secondary alkaline cells, and there is the disadvantage that when these materials are used in primary cells, the terminal voltage is lowered compared to cells containing nickel oxide alone as the positive electrode material. In addition, the discharge curve of the cell is not flat, but decreases to a lower voltage during discharge of the cell. Still another disadvantage of the cells with the nickel oxide-manganese dioxide electrode mixture is that only a very low capacity is available when the cells are used at lower temperatures.
A need therefore, exists for cells of improved structure which maintain a sufficient capacity even under heavy current loads, and which have an improved stability while being stored.